The present invention relates to an AC motor driving system for driving a multi-phase AC motor using a plurality of inverters, and more particularly, to a controller for controlling the inverters.
Generally, a motor driving system is comprised of a power converter and a controller for controlling the power converter. For realizing a large capacity motor driving system, it is necessary to increase the capacity of the power converter. One method of realizing a large capacity motor drive involves driving a set of plural power converters in parallel and supplying the sum of output power of the respective converters to an AC motor.
For operating a set of converters in parallel, there are two alternative methods. In one method, each converter is connected to a motor through a reactor or an interphase reactor. In the other method, a multi-winding motor is used such that one converter is connected to a set of windings of the multi-winding motor. The respective converters are electrically coupled in the former case, and magnetically coupled in the latter case. Due to the existence of such coupling, an unwanted circular current flows between the converters through this coupling if a voltage difference is caused by variations in switching characteristics (for example, a turn-off characteristic) of switching elements which form parts of the respective converters. This circular current is also called xe2x80x9cinter-inverter cross currentxe2x80x9d or simply xe2x80x9ccross current.xe2x80x9d In the following description, the designation xe2x80x9ccross currentxe2x80x9d will only be used.
A control method for effectively suppressing the cross current is described, for example, in JP-A-3-253293. The described method controls a current adjuster for controlling output currents of power converters such that a control gain for the sum of output currents of the respective power converters is able to differ from a control gain for an unbalanced current of each converter, i.e., a cross current. In this way, a control response to the cross current can be set independently of a control response to the sum of the output currents of the respective converters. Advantageously, the cross current can be suppressed with an appropriate control gain, and reactors with smaller inductance may be employed in consequence.
However, the characteristics and configuration of the control described in the foregoing prior art may be detailed as follows. [1] When the control gain for the sum of output currents is completely independent of the control gain for the cross current, the control configuration is complicated. [2] When there is a certain restriction between the control gain for the sum of output currents and the control gain for the cross current (for example, the latter can take only one half of the former), the control configuration is simplified.
Stated another way, with the control configuration described in the prior art, when an attempt is made to set the control gain for the sum of output currents independently of the control gain for the cross current, the control configuration inevitably becomes complicated. Therefore, if a microcomputer or a digital processor is used to execute the control, a longer time will be required for the processing. This results in a larger time delay and a reduction in stability margin, thereby giving rise to a problem in that the control gain cannot be increased and control response characteristics and disturbance suppression characteristics of the control system are degraded. Taking a motor driving system for an elevator as an example, a degraded disturbance suppression characteristic would cause increased high harmonic components in an inverter output current which act as torque ripple of a motor and bring about vibrations while the elevator is running.
Further, as another problem encountered when the control gain for the sum of output currents is set independently of the control gain for the cross current, the prior art control configuration suffers from difficulties in promptly switching to an individual inverter operation mode due to the complicated control configuration. For example, when one of two inverters operated in parallel fails, it is necessary to promptly switch the motor driving system to a mode in which the failed inverter is stopped, and the operation is continued with the remaining inverter. However, since the complicated control configuration described in the prior art results in a long time required for changing to the individual inverter operation mode, even the normal inverter is likely to end up in a failure due to an overcurrent or the like. In other words, the prior art control configuration is disadvantageous in that it is susceptible to a failure which has occurred in the system.
However, if the control configuration is simplified in order to avoid the two problems, a restriction imposed to the control gain for the sum of output currents and the control gain for the cross current would give rise to grave problems in that smaller reactors cannot be employed, the maximum output is reduced, and the power factor and efficiency are degraded.
It is an object of the present invention to provide a motor driving system in a simple control configuration which is based on the control principles that allow a control gain for the sum of output currents to be set independently of a control gain for a cross current. It should be noted that the sum of output currents is equal to a motor current, so that an expression xe2x80x9ca control gain for a motor current can be set independently of a control gain for a cross currentxe2x80x9d is used in the following description.
To achieve the above object, in a control processing system for deriving a control instruction for each of a plurality of power converters, the control processing system comprises a plurality of current compensation units connected in parallel. Each of the plurality of current compensation units makes compensation to reduce a deviation of detected output current values of the plurality of current converters from current instruction values to be outputted from the power converters to zero. Also, a control instruction is generated for the power converter by summing outputs of the parallel current compensation units associated therewith.
With the configuration of the control processing system as described above, a control for motor currents can be performed independently of a control for cross currents in principle provided that compensation gains of the current compensation units satisfy certain conditions. Further, because of a simple control configuration, even when the control is implemented by a microcomputer or a digital processor, the processing will not take a long time, so that a control gain can be increased. Also, by stopping some of the plurality of parallel current compensation units, a prompt switching to an individual inverter operation mode can be achieved.
According to the present invention, when a plurality of inverters are operated in sets in parallel configuration to drive one motor, a control processing system associated with each of the inverters is comprised of a plurality of parallel current compensators divided into local compensators and remote compensators. With this configuration, it is possible to independently carry out suppression of cross currents flowing between the inverters through electric coupling or magnetic coupling and appropriate control for motor currents. Additionally, since the control configuration is simple as compared with the prior art control configuration having similar effects, assuming that the control processing systems are implemented by a microcomputer or a digital processor, a processing time can be reduced, i.e., a time delay is reduced, thus leading to a larger stability margin of the control system, and a higher compensation gains of the compensators, as compared with the prior art. This results in improved instruction response characteristics and disturbance suppression characteristics of the control system, and further suppression of torque ripple occurring in the motor.